Imaging Thoracic Aortic Aneurysm

Introduction

Thoracic aortic aneurysm (TAA) is a chronic condition that manifests as progressive dilation of the thoracic aorta resulting from degradation of the normal smooth muscle cells and extracellular matrix proteins that provide integrity to the aortic wall. TAA can be broadly classified into 3 categories based on etiology: degenerative, genetically mediated and inflammatory (i.e., aortitis). Degenerative aneurysms are the most common, are associated with advanced age, occur in the absence of a defined genetic aortopathy or familial clustering, and are associated with cardiovascular risk-factors such as atherosclerosis and hypertension. Genetically-mediated TAAs are those which occur in the setting of a known clinical syndrome (e.g., Marfan, Ehlers-Danlos) or in the setting of a mutation in genetic mutations in molecular pathways known to be associated with TAA (e.g., TGF-β signaling pathway).¹

The prevalence of TAA has increased from 3.5 to 7.6 per 100,000 persons between 2002–2014.² In part, this is due to increasing rates of incidental detection on unrelated imaging studies (e.g., lung cancer screening, coronary CTA/calcium scoring). Incidental aortic dilation (>4.0 cm) in present in about 3% of patients > 55 years old.³ Maximal aortic diameter is currently the primary metric used to guide surveillance strategy and timing of surgical intervention for patients with TAA. As aortic diameter increases so does the risk of developing life-threatening complications, the most common of which is aortic dissection (i.e., delamination of the aortic wall) and less commonly rupture (i.e., transmural tearing). In the absence of acute complications, TAAs grow slowly over years or even decades, with typical growth rates in the range of 1–3 mm/year. When the aorta size reaches its biomechanical “hinge point”- usually about 6 cm in diameter- wall integrity rapidly declines, growth accelerates, and the incidence of complications rapidly increases. Current guidelines recommend surgical repair of the ascending aorta before the maximal diameter “hinge point” is reached, typically at a threshold of 5.5 cm.⁴ The primary management objective for TAA is to identify aortic growth early and to surgically replace the aorta before it reaches a high-risk size.

Non-invasive imaging surveillance plays a central role in the management of TAA though its ability to determine maximal aneurysm diameter and monitor for growth and other complications. Transthoracic echocardiography (TTE) can be used to monitor TAA that is limited to the root and proximal ascending aorta; however, computed tomography angiography (CTA) and magnetic resonance angiography (MRA) are the most commonly imaging modalities for evaluation of TAA as they can evaluate the entire thoracic aorta without the limitations of acoustic windows. Current guidelines generally lack detailed recommendations for the frequency of imaging surveillance and there is variations in approaches between physicians and centers, however, it is generally agreed that in degenerative TAA where the degree of dilation is mild or moderate (4.0–5.0 cm), annual follow-up imaging is appropriate and spacing to biennial or triennial if aortic dimensions have shown long-term stability.^5,6 When aortic dimensions are clearly increasing or approaching surgical thresholds, imaging frequency is typically increased to biannual.

Normal Aortic Anatomy

The thoracic aorta can be divided into the following regions: aortic root, ascending aorta, aortic arch, and descending aorta. The aortic root includes the annulus, aortic valve and sinuses of Valsalva. The conventional aortic anatomy consists of three sinuses corresponding to the aortic valve cusps (right, left and noncoronary). The three sinuses of Valsalva taper and form a “waist” at their junction with the tubular ascending segment (i.e., the sinotubular junction or STJ). The tubular ascending aorta extends from the STJ to the first arch vessel, and is so named given its lack of branches and resemblance to simple “tube”. Beyond the tubular segment, the aorta arch gives rise to the arch vessels (innominate, left common carotid, and left subclavian) from the proximal aortic arch. The distal arch beyond the left subclavian artery to the region of the ligamentum arteriosum is called the aortic isthmus. This region is of clinical significance, as it is a common site of non-fatal traumatic aortic injury and coarctation. The descending thoracic extends to the diaphragmatic hiatus.⁷ Guidelines suggest that aortic diameters be reported at specific aortic locations along the aortic length including the sinuses of Valsalva, STJ, mid-ascending aorta, proximal and distal arch, mid-descending aorta and at the diaphragmatic hiatus.⁵ Either sinus-to-sinus and sinus-to-commissure measurements may be reported for the sinuses of Valsalva.

Normal sizes for the thoracic aorta have been defined from several reference populations. In general, aortic size increases with patient age, male gender and body size.^8,9 However, measurement techniques can introduce variability into the reported size of the thoracic aorta. These include measuring the aorta using gated versus non-gated imaging technique (and when gated, during systole versus diastole), from inner versus outer edge, and in the axial versus double-oblique planes. The section below will explore best practices of measurement technique.¹⁰

The range of mean ascending aortic diameters (including gated and non-gated exams) in the literature by CT ranges from 29.0–37.2 mm for females, and 30.8–39.1 mm for males, with the larger diameters reported for studies without electrocardiographic (ECG)-gating.¹¹ While in general it is accepted that the maximal diameter of the ascending thoracic aorta should be below 40 mm in healthy individuals,⁶ some series have shown that the normal range (within two standard deviations of the mean) for males and females can extend above this level. Considering the significant impact of patient size on normal aortic diameter, indexing aortic dimensions to adjust for patient body size (i.e., height or body surface area) is appropriate for optimal definition of pathologic aortic dilation; however, clinical application of indexed aortic measurements in adults is limited due to the lack of comprehensive population nomograms to determine reference ranges.

Imaging Technique

When selecting an imaging technique, the strengths and weaknesses of various imaging modalities should be considered in relation to the clinical context. The American College of Radiology (ACR) Appropriateness Criteria for TAA initial imaging rates both CTA and MRA as “usually appropriate”¹². For pre-procedure planning prior to thoracic endovascular repair (TEVAR), CTA chest, abdomen, and pelvis is rated at 9 “usually appropriate” while MRA and CTA chest alone are rated at 7 “usually appropriate.” CTA is often preferable to MRA following TEVAR given the increased artifact as a result of metal stent (particularly those with stainless steel) as well as the increased ability of CTA to detect post-operative infection. Pros and cons of CTA versus MRA are summarized in Table 1.

Table 1:

Pros/Cons of imaging modalities

Table: Pros and Cons of CTA vs. MRA
Characteristic	CT Angiography (CTA)	MR Angiography (MRA)
Radiation	Ionizing radiation (X-Ray) + DNA Damage	Non-ionizing (Radiofrequency) No DNA Damage
Spatial resolution (typical)	0.5 – 1.5 mm3	0.7 – 1.5 mm3 (variable)
Number of acquisitions	Usually single	Usually multiple
Set-up and Scan Time	Short (5–10 min)	Long (45–60 min)
Acquisition Complexity	Easy	More Difficult
Patient Participation	Minimal	Significant – Multiple breath hold
Modality Strength	Anatomy and Post-surgical evaluation	Soft Tissue Characterization and Hemodynamic/Functional Assessment
Contrast Risk	Iodinated Contrast: Contrast-Induced Nephropathy (CIN) -Rare Severe Allergy: ~1:1,000	Gadolinium Contrast: Nephrogenic Systemic Fibrosis (NSF) -Extremely rare Gadolinium deposition in brain (unclear clinical significance) Severe Allergy: ~1:100,000

Measurement Techniques

Measurement techniques can introduce significant variability into the reported size of the thoracic aorta. Different measurement techniques utilized in clinical practice by different centers has been shown to result in a lower reproductivity for CT compared to echocardiography.¹³ One method to reduce this variability is through the utilization of double-oblique or orthogonal measurements. Double-oblique measurement obtained orthogonal to the aortic centerline allows creation of a true short axis reformation of the aortic diameter and has been shown to allow more accurate measurement of aortic size compared to axial measurement (Figure 1). Axial measurement may result in a significant overestimation of aortic size, up to 6mm or 21% increase in size according to Hager et al.⁸ In one series, axial measurements were shown to overestimate aortic size at multiple locations (with the exception of the aortic arch) and resulted in the misclassification of 13% of patients into either aneurysmal or surgical candidate categories (Figure 2).¹⁴ It is also important to recognize that measurement of inner to inner, leading edge, or outer to outer diameter of the aorta can also introduce variation in aortic size.¹⁵ Consensus as to which of these methods is preferred has not been established for CT and MR, although leading edge to leading edge is a frequent standard utilized with echocardiography. Within a center, consistent technique should be adopted to decrease measurement variability between serial scans.

Figure 1:

Double oblique measurement technique of the aortic arch and 3D reformation of the thoracic aorta in a patient with connective tissue disease undergoing routine surveillance.

Figure 2:

Axial contrast enhanced CT depicting aortic measurement perpendicular to the aortic axis (yellow arrow) versus overestimation of aortic size when measurement is obtained parallel to aortic axis (blue arrow).

Imaging Protocols

The thoracic aorta is best evaluated with cross-sectional imaging, either computed tomography (CT) or magnetic resonance (MR). While computed tomography angiography (CTA) and magnetic resonance angiography (MRA) imaging techniques are routinely utilized to evaluate the aortic size and structure, specific CT and MR imaging protocols can be additive in evaluating thoracic aortic pathology.

Computed Tomography

The standard multidetector CT evaluation of TAA consists of contrast enhanced CTA. Non-contrast CT may be obtained prior to CTA in order to assess for intramural hematoma in the setting of concern for acute aortic syndrome or to assess for calcification or surgical material in a postoperative patient. It is important to distinguish aortic wall thickening resulting from atherosclerosis, which presents as circumferential aortic wall thickening that is stable over time, from acute intramural hematoma which tends to be eccentric in location (Figure 3). Contrast enhanced CTA of the aorta may be performed with bolus tracking or utilization of a timing bolus in order to ensure optimal enhancement of the thoracic aorta. Post contrast delayed phase may also be obtained in patients with endovascular repair of TAA or dissection (TEVAR) to assess for endoleak or in patients with inflammatory TAA/aortitis to evaluate for peri-adventitial enhancement indicating active inflammation.

Figure 3:

A) Axial non-contrast CT of the chest in a patient with acute intramural hematoma.

B) Axial non-contrast CT of the chest in a patient with aortic wall thickening as a result of atherosclerosis.

C) Follow-up CT of the second patient 6 months later demonstrates stable findings.

ECG-gating, either prospective or retrospective may be obtained, and if done, care should be made to compare aortic measurements at equivalent phases of the cardiac cycle on subsequent examinations. Benefits of ECG-gated analysis of the thoracic aorta include a reduction in motion artifact, which can be particularly helpful when evaluating the aortic root/ascending aorta or in cases of suspected aortic dissection as a complication of TAA (Figure 4). ECG-gated CTA also can allow concurrent assessment of the proximal coronary arteries. Roos et al demonstrated that both prospective and retrospective ECG-gating reduced motion artifacts throughout the thoracic aorta, although the greatest improvement in image quality was noted at the level of the aortic valve.¹⁶

Figure 4:

A) Non-gated axial contrast enhanced chest CT depicting motion related artifact at the aortic root simulating aortic dissection (yellow arrow). B) Repeat CT with cardiac gating shows resolution of the artifact.

In general, aortic size is slightly larger in systole compared to diastole¹⁷. Mao et al demonstrated that the ascending aorta measured 1.7 mm larger at end systole (35% of the R-R interval) compared to end diastole¹⁸. de Heer also demonstrated larger diameters in systole for the aortic valve annulus, sinuses of Valsalva, and STJ, in the range of 0.4–1.0 mm difference. CTA prior to transcatheter aortic valve replacement (TAVR) have shown an 8% higher annulus area in systole, with greater variability in size for those patients without significant annular calcification^19,20. Diastolic images are most commonly used for aortic measurements to minimize motion artifact.

Magnetic Resonance

A unique benefit of MRI is the ability to perform high-quality vascular imaging without the need for potentially nephrotoxic contrast agents and without ionizing radiation exposure. Disadvantages of MRI include the slightly lower spatial resolution compared to CT, reduced ability to evaluate burden of calcified atherosclerosis, artifacts in the setting of metallic stents or implants and the potential for patient claustrophobia. A variety of sequences can be performed to evaluate both the aortic wall and lumen (black blood spin echo or fast spin echo imaging), aortic valve morphology (steady state free precession cine), and valve function (phase contrast imaging).²¹ Magnetic resonance of the aorta is most commonly performed using gadolinium-based contrast agents although iron-based agents (e.g., ferumoxytol) are being increasingly utilized given their high relaxivity and long intra-vascular half-life.²² Gadolinium-enhanced MRA is typically performed in arterial phase, after bolus injection, although slow continuous infusion protocols have been described.²³ A significant advantage of MRI is the ability to perform non-contrast MRA using steady state free precession (i.e., “white blood”) techniques, with comparable measurement quality compared to contras-enhanced techniques.²⁴ Three-dimensional non-contrast MRA techniques have also been shown to be feasible for evaluation of the aortic annulus prior to transcatheter aortic valve replacement, including respiratory and cardiac gating.²⁵

Multiplanar and Three-dimensional Analysis

Beyond the benefits of multi-planar reformats for aortic measurements (discussed above), an additional benefit of both CTA and MRA is the ability to easily create three-dimensional reconstructions of TAA. Volume rendered three-dimensional reconstructions can be easily generated using variety of clinical 3D image analysis software tools, and can be useful in providing a visual overview of the extent of disease, especially during procedure planning. Additionally, the volumetric nature of CTA and MRA data is well suited for 3D printing, which allows rapid creation of anatomic models. 3D printed models allow clinicians, surgeons, and patients the opportunity to comprehend complex cardiovascular anatomy and pathology in a more tangible manner than achieved with computer-based 3D modelling, and may assist in planning endovascular or operative repair.²⁶

Safety and Quality

There has been increased focus on the risks associated with radiation exposure in medical imaging. This risk is particularly relevant for TAA patients who often undergo repeated imaging as a part of imaging surveillance, and may be subjected to a significant radiation doses over their lifetime. Zoli et al evaluated the cumulative radiation dose exposure of patients with TAA who underwent TEVAR and subsequent follow-up, and identified that these patients would be exposed to 89mSv over one year adhering to an institutional surveillance protocol, with a lifetime radiation exposure of greater than 350 mSv, and concluded that this was associated with a lifetime increased risk of malignancy of 2.7%.²⁷ Eliminating non-essential sequences, such as non-contrast and delayed phase CTA, when not clinically indicated can significantly reduce radiation doses. Retrospective ECG-gating is associated with a higher radiation dose compared to non-gated examinations, often nearly 2-fold greater.²⁸ Other scanner and protocol related methods such as tube current modulation, iterative reconstruction, and dual energy CT (DECT) can substantially reduce patient radiation dose.²⁹

Recent studies have questioned the causal relationship of iodinated contrast and acute kidney injury, as discussed in the ACR Manual on Contrast Media 10.3.³⁰ However, the possibility of contrast-induced nephropathy and post-contrast kidney injury remain a consideration in the selection of aortic imaging modality. Vascular imaging techniques that reduce or eliminate the use of iodine based intravenous contrast (such as magnetic resonance imaging) may be preferable, particularly in patients with pre-existing renal impairment.

Current literature supports the safety of group II gadolinium-based contrast agents in patients with chronic renal insufficiency, a common comorbidity in patients with TAA. While nephrogenic systemic fibrosis (NSF) remains a concern in clinical practice, few if any, non-confounded cases of nephrogenic systemic fibrosis have been seen with group II agents. Ferumotyxol has emerged as an alternative to gadolinium-based agents given its safety in renal insufficiency and lack of heavy-metal deposition in the body. Ferumoxytol is an ultra-small superparamagnetic iron oxide (USPIO) particle approved by the Food and Drug Administration (FDA) for use in patients with chronic kidney disease for the treatment of iron deficiency anemia, that can be used off-label as an MRA contrast agent with high relaxivity and long intravascular half-life (14–21 hours).

Findings, Pathology and Diagnostic Criteria:

Aneurysm Morphology, Etiology and Extent

While maximal diameter is the primary metric used to guide patient management, there are other features of aneurysm morphology that give insight into potential aneurysm etiology and are important for surgical planning. The first step in assessing TAA is determination of the dilated segment (ascending or descending). Occasionally both the ascending and descending segments are dilated, although typically dilation is most severe at one segment. Dilation of the aortic arch is can be seen in association with ascending or descending TAAs, however, usually the arch dilation is contiguous with an adjacent ascending or descending aortic aneurysm and the degree of arch dilation is lesser than the primary aneurysm. Isolated aortic arch aneurysms are uncommon, and are strongly associated with atherosclerosis. Beyond simply describing TAA anatomy, there are clear associations between the location of aneurysm and the etiology. Recent research has shown that embryologic origin of aortic smooth muscle cells varies by aortic segment, suggesting that aneurysm morphology may be related heterogeneity in smooth muscle cell distribution, especially in genetic aortopathy.³¹

Another key distinction in aneurysm morphology is whether the aneurysm is fusiform or saccular. Fusiform (“spindle-shaped) morphology describes and aneurysm that gradually tapers at each end of the aneurysm to a relatively normal diameter, and with the distribution of wall dilation being relatively circumferential. The fusiform morphology is most indicative of a true aneurysm (i.e., all three aortic wall layers are intact), and is most commonly seen with degenerative and genetically-mediated TAAs. Conversely, saccular aneurysms tend to have more abrupt transition points with the adjacent uninvolved aorta, present as a more discrete and eccentric outpouching of the aortic wall, and often have an area of narrowing at their aortic attachment (the “neck”) which leads to the body the aneurysm. A saccular aneurysm morphology is most suggestive of aortic pseudoaneurysm (i.e., less than three aortic wall layers remain intact), and common etiologies include penetrating atherosclerotic ulcer (PAU), mycotic/infectious, iatrogenic and post-traumatic.

Key imaging findings that suggest specific etiology and risk

While there is significant overlap in TAA morphology between different subgroups of TAA there are imaging findings that suggest specific etiologies, risks and are important to consider when planning treatment strategies. Fusiform dilation of the ascending aorta is most commonly associated with degenerative TAA, however, can also be seen with genetically mediated TAA. When aortic dilation predominantly affects the aortic root, aortic-related connective tissue disorders (CTD) such as Marfan syndrome (mutated fibrillin-1 gene), should be strongly considered. This pattern of aortic root dilation is termed “annuloaortic ectasia” (AAE) (Figure 5A), and is classically associated with Marfan syndrome and describes proximal aortic dilation that involves the annulus, sinuses and proximal ascending aorta, resulting in loss of the normal “waist-like” contour of the sinotubular junction (STJ). Annular dilation often results in substantial aortic insufficiency and thus surgical repair of AAE must not only repair aortic root dilation but also re-size the aortic annulus (i.e., annuloplasty) if the native aortic valve is to be preserved. Vascular Ehlers-Danlos syndrome (vEDS) is a specific aortic-related connective tissue disorder caused by mutations in the COL3A1. vEDS leads to the development of arterial aneurysms at a young age, often resulting in rupture/dissection. Imaging manifestations of vascular EDS are similar to that of Marfan syndrome, and can include TAA (often with AAE) as well as ectasia and dissection of the principal aortic branches (e.g., visceral aneurysms).³² Loeys-Dietz syndrome (LDS) is an extremely rare connective tissue disorder that results in aortic abnormalities similar to Marfan and vEDS, although patients with LDS generally considered the most severe of the genetically mediated TAAs and complications present at early ages and smaller aortic diameters. While imaging findings in LDS also include ascending aneurysm (often with AAE) and dissection, a characteristic unique LDS that is severe tortuously of the vertebral arteries, which has been shown to correlate with increasing torts will also be shown to be correlated with severity of disease course (Figure 5B). Given the variable severity of disease in aortic-related CTDs, different maximal aortic diameter thresholds have been used to trigger surgical repair with the most aggressive repair thresholds of 4.0–4.5 cm for repair in patients with LDS and threshold in the range of 4.5–5.0 for vEDS and Marfan syndrome.¹ While most CTDs have other non-vascular manifestations (e.g., musculoskeletal, craniofacial, cardiac) aortic aneurysm is a leading cause of morbidity and mortality in these populations, specifically because of the substantially higher rates of aortic dissection compared to the general population.³³ While not classically considered a CTD, patients with bicuspid aortic valve experience TAA and dissection at a significantly higher rate than the general population and experience accelerated degeneration of their aortic wall integrity, a phenomenon termed bicuspid valve aortopathy. Patterns of ascending TAA are variable, but most commonly there is pronounced dilation of greater curvature (the convexity) of the ascending aorta in BAV (Figure 5C). A smaller proportion of BAV patients demonstrate a root-dilation phenotype and have clinical characteristics more similar to classic aortic-related CTDs.³⁴

Figure 5:

A) Coronal reformat demonstrating the typical shape of annuloaortic ectasia in a patient with Marfan

B) Sever tortuosity of the vertebral arteries (yellow arrow) as seen in Loeys-Diets syndrome.

C) Bicuspid aortic associated ascending TAA with maximal dilation at the mid-ascending level along the greater curvature (asterisks)

Inflammatory disease of the thoracic aorta can result in aortitis and secondary TAA. Classic imaging findings of aortitis include thickening of the aortic wall (often best seen by MRI) as well as periadventitial enhancement and edema, often resulting in stranding of the periaortic fat on imaging.³⁵ The wall thickening of aortitis is typically relatively circumferential, which can help differentiate from aortic wall thickening related to atherosclerosis, which is largely eccentric. Extensive intimal calcification can be seen in chronic (“burned out”) aortitis, and also tends to be more diffuse and circumferential than typically seen with atherosclerosis. FDG PET/CT can be useful for confirming aortitis in depicting the full extent of involvement, with areas of active disease typically demonstrating very avid FDG uptake (atherosclerotic disease typically shows low-level or no FDG uptake). The two most common forms of non-infectious aortitis include Takayasu arteritis and giant cell arteritis (GCA), which can usually be easily differentiated in patients given the different populations they affect; Takayasu is seen in young (<40) women and GCA is seen in older patients (>50) and is associated with polymyalgia rheumatica and temporal arteritis. Both Takayasu and GCA can result in TAA, but a unique feature of Takaysu is the propensity for the aortic wall inflammation that in the chronic phase results in medial fibrosis and progressive stenosis, most commonly affecting the proximal aortic arch vessels, although involvement of the entire thoracoabdominal aorta and its principle branches can be seen. Infectious causes of aortitis are rare in the developed world and are usually seen either in the setting of endocarditis/bacteremia resulting in development of an infectious nidus within the aortic wall (typically in an area of atherosclerotic plaque), or in the postoperative/iatrogenic setting.

Noninfectious, inflammatory atherosclerotic plaque results in a spectrum of abnormalities that may result in aortic pseudoaneurysm. The earliest manifestation of inflammatory atherosclerotic plaque is termed ulcerated plaque, where there is erosion of the fibrous of the aortic atheroma, but without violation of the internal elastic lamina (IEL), which separates the intima from the media). Once inflammatory plaque erosion extends through the IEL, the process is pathologically termed a penetrating atherosclerotic ulcer (PAU). While the IEL is not able to be resolved by imaging, it is typically assumed that if there is bulging of the outer aortic wall/adventitia on imaging that the IEL is disrupted and a PAU is present. Small PAUs are commonly seen on contrast-enhanced imaging of the chest, and while there is a significant lack of data regarding the natural history and associated risks with small PAUs, they are generally thought to have an indolent course and the minority result in complications (<15%) or significant aortic enlargement (~25%).³⁶ Specific surveillance guidelines for small PAUs are lacking, but typical management involves imaging surveillance with CTA or MRA. When PAUs extend significantly (>2 cm) beyond the aortic wall and assume a more saccular morphology, they are often termed pseudoaneurysms, and have a much more aggressive disease course characterized by rapid growth and development of complications (e.g., rupture). Aortic pseudoaneurysm are typically managed surgically, and most commonly using endovascular techniques in the descending thoracoabdominal aorta.

Imaging Features and Findings That Imply Risk

A comprehensive imaging assessment of TAA requires accurate description of secondary findings that imply elevated risk of complications, as such features could significantly change the patient’s course of treatment. While not an imaging feature, pain is a key symptom in aortic disease, and patients presenting with chest pain thought to be related to TAA, PAU or other aortic abnormality are generally treated with surgical repair regardless of aortic diameter and imaging features. Other important high-risk imaging features include:

1. Interval growth is an important feature of TAA imaging assessment that needs to be accurately assessed and described when multiple imaging studies are available. TAA growth rates are highly variable, ranging between 0.2 to 4.2 mm/y.³⁷ Growth rates of >5 mm/y are considered an indication for surgical repair in current AHA guidelines⁵, although in practice a 10 mm/y rate is more commonly used given the substantial variability in diameter measurements. Accurate measurement of TAA growth requires careful attention on the part of the radiologist to ensure that measurements are made in a similar fashion between multiple studies, ideally using multi-planer reformats.

2. Intramural hematoma (IMH) is a cause of acute aortic syndrome and is characterized by a hematoma that forms within the medial layer of the aortic wall. IMH is considered a dissection variant, is classified similarly, and typically requires urgent surgical repair.

3. Aneurysm rupture or leak is most commonly seen in descending thoracoabdominal aneurysms and is uncommon in ascending TAA without associated type A dissection. CT findings of rupture/leak include: aortic fat stranding, high-density periaortic fluid collection, pleural/pericardial effusion (often hemorrhagic), and rarely aortic contrast extravasation.

What the Referring Physician Needs to Know

Thoracic aortic aneurysm is a surgical disease as there are yet to be any proven medical treatments to slow disease progression or prevent aortic complications. Thus, imaging evaluations need to aid in the determination of surgical candidacy and inform repair strategy if surgical indications are present. The imaging report in TAA should include the following important features:

1. Maximal diameter is the primary metric used to determine TAA management. It is of utmost importance that imagers accurately measure and report maximal aortic diameter in all patients with TAA, and diameter measurements should be performed using double-oblique reformats rather than on axial images to improve measurement accuracy and minimize measurement variability. When multiple imaging studies are available, it is important for imagers to assess and report TAA stability, ideally performing measurements of maximal diameter with the same measurement technique over multiple prior studies if available.

2. Extent of the aortic disease has important implications for surgical management. Specifically, in ascending TAA description of whether aortic dilation involves the root and/or arch has important implications for surgical given that the repair of these statements requires specific surgical techniques, more challenging surgical techniques, and imply additional surgical risks. In descending TAA, it is helpful to report the proximity of the dilated descending thoracic aorta to the left subclavian artery, as this will determine the availability of a proximal landing zone for endovascular repair, as well the distal extent of the aneurysm, as involvement of the abdominal aorta and visceral vessels implies additional technical demands and risks.

3. Imaging assessment of the aortic valve can be useful in informing patient management as concomitant aortic valve disease is not uncommon with TAA. Specifically, it should be noted if there is a significant degree of aortic valve leaflet calcification (raising the possibility of aortic stenosis), and if image quality permits assessment of aortic valve morphology, it is important to note evidence of bicuspid aortic valve. Bicuspid aortic valve is often better characterized by CT than echocardiography (ref), and is associated with higher risk of progressive TAA growth and dissection.

4. A qualitative assessment of atherosclerotic severity, nature (calcified vs. non-calcified) and distribution can be useful for guiding aggressiveness of medical therapy and can inform the risk of at atheroembolic stroke during surgical repair.

5. The presence of secondary findings that imply heightened risk, as described above (e.g., PAU, IMH, rapid growth), should be explicitly conveyed in the Impression section of the report.

Summary

High quality aortic imaging plays a central role in the management of patients with TAA. CTA and MRA are the most commonly utilized techniques for TAA diagnosis and imaging surveillance, with each having unique strengths and limitations that should be weighed when deciding patient-specific applications. While there are a wide range of potential etiologies for TAA (genetic, degenerative and inflammatory), the majority of these diseases are managed based on measurement of maximal aortic diameter. To ensure optimal patient care, imagers must be familiar with potential sources of artifact and measurement error, and dedicate effort to ensure high quality and reproducible aortic measurements are generated. A complete imaging report should not only describe aortic dimensions, but also provide a complete description of the extent of the disease, detail morphologic features that suggest a specific TAA etiology, and emphasize secondary imaging features that imply additional risk.

Synopsis:

This review will summarize the imaging evaluation and underlying pathology relevant to the diagnosis of thoracic aortic aneurysm.

Key Points:

• Cross-sectional imaging (CTA and MRA) plays a central role in management of patients with thoracic aortic aneurysm

• Maximal aortic diameter is the primary metric used to estimate risk and determine the need for surgical repair, although diameter measurement are subject to error related to image artifact and measurement technique.

• Optimal imaging surveillance requires selection of imaging modality (CTA vs. MRA) based on patient specific characteristics and indications, in addition to consistent measurement protocols based on double-oblique images to minimize measurement error.

• The majority of TAAs are classified as degenerative and associated with fusiform dilation of the ascending aorta, whereas root aneurysms are typically seen in aortic-related connective tissue disorders and descending thoracoabdominal aneurysms are strogly associated with atherosclerosis.